Innate immunity is the first line of defense designed to protect the host from invading pathogens, including HIV. We have previously described several natural killer (NK) cell dysfunctions in HIV-viremic individuals. In the past year, we investigated the effect of HIV envelope glycoproteins (gp120) on the physiologic functions of NK cells. Upon treatment of NK cells with HIV gp120, DNA microarray analyses indicated up-regulation of several categories of genes that are associated with apoptosis and suppression of both cellular proliferation and survival, as well as down-regulation of genes that play a vital role in cell proliferation, innate immune defense mechanism, and cell survival. Each of two subtypes of gp120, which bind to either the CXCR4 or CCR5 co-receptor, suppressed NK cell cytotoxicity, proliferation and ability to secrete interferon-alpha. NK cells exposed to X4-subtype HIV gp120 showed a significant decrease in the levels of CC-chemokines, while exposure to R5-subtype HIV gp120 had minimal effect. Extended exposure to HIV gp120 resulted in apoptosis of NK cells, further validating the microarray data. Our data demonstrate that exposure of NK cells to HIV envelope proteins results in profound cellular abnormalities at the level of gene expression as well as generic cell functions. These findings are likely to be a consequence of a direct HIV gp120-mediated effect on NK cells. Identification of specific surface receptors on NK cells that interact with HIV envelope proteins might explain how HIV circumvents innate immune defense mechanisms and establishes infection in susceptible individuals.[unreadable] We also performed DNA microarray analyses as well as phenotypic and functional analyses using NK cells from HIV-infected individuals in an effort to elucidate the mechanisms by which ongoing HIV replication affects the physiologic function of these cells. Genetic profiles (DNA microarray) and phenotypic and functional characteristics of NK cells isolated from HIV-infected viremic and aviremic, as well as HIV-seronegative individuals, were compared. More than 100 genes were shown to be up-regulated in NK cells from HIV-infected viremic individuals when compared to those from HIV-infected aviremic and HIV-negative individuals. Several of those genes belong to the immune response and apoptosis gene families. Functional assays confirmed an increased propensity of NK cells from HIV viremic individuals to undergo Fas-mediated apoptosis (FMA). Furthermore, increased expression of CD95 on NK cells of HIV-infected viremic individuals was associated with increased susceptibility to undergo FMA, but not CD16- or NKG2D-mediated apoptosis. Serum levels of sFasL and expression of Ki67 on NK cells were markedly elevated in HIV-infected viremic individuals when compared to those of HIV-infected aviremic and HIV-negative individuals. [unreadable] Our data demonstrate that ongoing HIV replication results in profound NK cell abnormalities that are likely to be due to the effects of virus-induced immune activation. Of note is an increased susceptibility to cell-death mediated by CD95-sFasL interactions. In addition, these NK cells, particularly the CD56dim CD16bright subset, undergo enhanced cell turnover in vivo as demonstrated by intracellular Ki67 expression.[unreadable] Recent studies have shown that stimulation of NK cells using TLR-9 agonists can activate NK cells and provide protective immunity against herpes virus infection. It has been speculated that this activation of NK cells probably involves plasmacytoid dendritic cells (pDCs), which are directly stimulated by TLR-9 agonists. Recently we investigated the role of HIV viremia and HIV proteins on NK cell-pDC interactions. TLR-9 ligand (CpG) treatment of PBMCs activated NK cells in a process mediated mainly by IFN-alpha and TNF secreted by pDCs. However, NK cells from HIV-infected individuals are less responsive to CpG than NK cells from healthy individuals. HIV gp120 exerts a profound suppressive effect on CpG-mediated activation of NK cells by CpG-stimulated pDCs. CpG-mediated activation of NK cells from HIV-infected individuals is probably due to a non-responsiveness of NK cells to the cytokines IFN-alpha and TNF and possibly due to an impaired ability of pDCs to secrete IFN-alpha and TNF. Studies using HIV gp120 demonstrate a profound direct suppressive effect of gp120 on both NK cells and pDCs, disrupting the cross-talk between these two cells. Future studies are required to determine the underlying mechanisms of gp120-mediated suppression of pDC-NK cell interactions.